In the field of radio receivers, there has been a concentrated effort to reduce the amount of tuned circuitry used in the receivers. By reducing the number of tuned circuits, large portions of the receiver can be integrated resulting in smaller receivers. These compact receivers can then be used in many areas such as cellular telephones. A major advance in the design of such receivers is a technique known as the "zero-IF" technique.
Slope, drift, and offset compensation in zero-IF receivers is discussed, for example, in U.S. Pat. No. 5,568,520 entitled "Slope Drift and Offset Compensation In Zero-IF Receivers". In addition, U.S. Pat. No. 5,241,702 entitled "Slope Drift And Offset Compensation In Zero-IF Receivers" discusses reducing DC offset in homodyne (zero-IF) receivers by differentiating the I,Q signals before digitization using an analog differentiating circuit, and then re-integrating the signal samples numerically after digitization to restore the signal's undifferentiated waveform. Each of these patents is hereby incorporated herein in its entirety by reference. Furthermore, the inventor of the present invention is an inventor in each of these patents.
In homodyne receivers, received signals are converted directly down to zero frequency I and Q signals, also called the quadrature baseband, by mixing the received signal against a local oscillator placed in the center of the desired receive channel. Since the local oscillator is directly on top of the received signal, it may be a significant source of interference to reception. Because the interfering signal may be the same as the downconversion oscillator, however, it is coherent interference and appears as DC offset on the I,Q outputs. This DC offset can be much larger than the weakest signal that the receiver is desired to receive and can drive the I,Q Analog-to-Digital convertors to full scale or beyond, causing signal degradation.
The patents discussed above thus provide ways to reduce DC offset of the I,Q signals to reduce interference from a local oscillator within a homodyne receiver. In known homodyne receivers, the local oscillator frequency may be the dominant crystal-related interfering signal because the local oscillator is tuned to the channel frequency by means of a digital frequency synthesizer using the crystal as a reference.
These patents, however, may not resolve problems relating to superhetrodyne receivers wherein received signals are mixed with a local oscillator that is not tuned to the channel frequency but that is tuned to the channel frequency plus or minus a constant offset equal to the desired first intermediate frequency. In such a system, the local oscillator may not be a significant source of interference to a desired signal and the problem of DC offset in I and Q signals may not be evident or obvious.
Interference to a desired signal by other crystal related frequencies such as crystal harmonics, however, may also contribute to the DC offset in the I and Q signals. Accordingly, there continues to exist a need in the art for improved receivers and methods of reducing interference.